Terrassenschotter des Neckars

Gravel deposits can be found in several places along the Neckar. Some of these are located a few metres above the current water level of the Neckar. In Klingenberg you can see them on a 300m long and 5m high rock face. How the gravel was created and how it got there is the subject of this Earthcache.

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Fig.1 – Detail of the Nagelfluh

Gravel

In the geosciences, gravel is understood to be deposits of coarse rock components that have previously been rounded off by their rolling transport in flowing waters or in the surf on coasts. They therefore no longer have any sharp edges. Between a diameter of 2 and 63mm one speaks of pebbles, above it of cobbles, below of sand and again below of silt.

1) Question:
At stage 1 you come close to the rock on the stairs and can see its composition well. Does the rock consist exclusively of rounded gravel or are there also stones with sharp edges and why is that? Do you mainly recognise pebbles or cobbles?

Pleistocene

The gravel of the Neckar is rock deposits from the Pleistocene. The Pleistocene is an ice age from 2.6 million years ago to 11,700 years, which ended with the transition to the warm age that continues today, the Holocene. The Pleistocene was characterised by an alternation of several warm and cold phases.

Gravel Terraces

The gravel of the Neckar was mainly formed in the cold periods of the Pleistocene, when weathering through frost blasting made a lot of broken rock material that could be transported. The gravel has been deposited in several so-called high-terraces within today's river valleys. They differ from the even older highest-level gravel, which can be found widely scattered above today's valleys. The widespread distribution of the gravel outside the river valleys suggests that the Neckar used to have a very flat, sweeping river system. The landscape in the Heilbronn area then changed between the period when the highest-level and high-terrace gravel was deposited. The Neckar and its tributaries had already cut a little deeper into the subsurface due to the much more powerful erosion of the flowing waters during the cold periods of the Pleistocene, and so there was no longer an expansive river system. The gravel was only deposited in a narrow area.
The sedimentation in terraces arose when the river cut into its own, previously heaped gravel bed under the rather cool and very humid conditions at the beginning and at the end of a cold period. The remnants of this gravel bed then often remained as terraces at the valley edges. Since these processes were repeated several times with the alternation of warm and cold periods during the Pleistocene, a total of up to 14 river terraces are counted today in the central Neckar valley. The youngest of them from the last glacial period, the so-called lower terraces of the Würm glacial period, are less than 5m above today's floodplain. On the other hand, the oldest terraces from the late Neogene period with the already mentioned highest-level gravel are up to 150m above.
In contrast to the older highest-level gravel, limestone pebbles from the Upper Jura also appear for the first time in the high-terrace gravel. At the time of its deposition, the Neckar had already been able to expand its catchment area far to the southeast and was already drawing some of its rock load from the area of ​​today's Swabian Alb. In the Heilbronn area in particular, he deposited the material he carried with him particularly strongly. Responsible for this is the so-called Heilbronner Mulde, a subsidence area through which the Neckar flowed in a widely ramified system at the time. In its centre near the Frankenbach district, the up to 24m thick layers of high-terrace gravel were mined in several pits - including the Ingelfinger gravel pit (reference point R1) - until the 1990s (see also GC9AW49). The gravel is also known as "Frankenbacher gravel" and contains many fossils from the same era as the famous Homo Heidelbergensis.

Fig.2 – above Klingenberg

Nagelfluh Rocks near Klingenberg

Above the houses of Klingenberg, up to 5m high cliffs protrude from the hillside above the vineyards. It is high-terrace gravel that has been baked into a concrete-like so-called conglomerate using a calcareous binding agent (calcareous seepage water). Gravel deposits that have solidified to form conglomerates are also specifically referred to as Nagelfluh. The Nagelfluh rocks have been declared a 0.4 hectare protected area in 1937. The exact period when the gravel was deposited is not known. However, it will be similar to that of the more closely examined layers in the "Frankenbacher Schotter" nature reserve three kilometres away (reference point R1). The layers of sand and gravel there were deposited in the older half of the Middle Pleistocene, i.e. in the period from 783,000 to 400,000 years ago. In addition to the Klingenberger Nagelfluh rocks, there are also six smaller Nagelfluh rocks on the opposite bank of the Neckar (reference point R2).

2) Question:
Go up the stairs to stage 2. Here you can see the Neckar below you. How deep has the Neckar cut into the valley since the Klingenberger Nagelfluh rocks were formed? Tip: the stairs cover about 3/4 of the height difference and one step is about 18cm high.

Animal Life

The sunny rocks are an important biotope for numerous animal and plant species. 75 different wild bee species, some of which are very rare, were found here. They rely on food plants such as upright ziest (Stachys recta), adder's head (Echium vulgare) or mignonette (Reseda lutea). Also noteworthy are the occurrences of the endangered wall lizard (Podarcis muralis) and the ant lion (Euroleon nostras).

References

[1] W. Hansch, s. Mailänder, W.-D. Riexinger, W. Rosendahl, T. Simon, "Frankenbacher Schotter", Sep 2006, ISBN: 978-3-00-019821-2
[2] Landesamt für Geologie, Rohstoffe und Bergbau Freiburg, https://www.lgrb-bw.de/geotourismus/gto/details.html?vid=9215, 13. Mai 2021
[3] Landesanstalt für Umwelt Baden-Württemberg, https://rips-dienste.lubw.baden-wuerttemberg.de/rips/ripsservices/apps/naturschutz/schutzgebiete/steckbrief.aspx?id=1219002000006, 13. Mai 2021
[4] Landesanstalt für Umwelt Baden-Württemberg, https://rips-dienste.lubw.baden-wuerttemberg.de/rips/ripsservices/apps/naturschutz/schutzgebiete/steckbrief.aspx?id=1219002000003, 13. Mai 2021
[5] H. Brunner (1986), mit Beitr. v. K.D. Adam, S. Müller, T. Simon & H. Wild: Erläuterungen zu Blatt 6821 Heilbronn. Geol. Karte Baden-Württ. 1:25000. – 3. Aufl., 204 S., 4 Beil.; Stuttgart.
[6] E. Bibus & J. Wesler (1995): The middle Neckar as an example of fluvio-morphological processes during the Middle and Late Quarternary period. – Z. Geomorph., N.F., Suppl. Bd. 100: 15-26; Berlin, Stuttgart.